The developmental pattern of phenylalanine hydroxylase was studied in rat kidney. Traces of activity were observed on the 20th and 21st days of gestation, but significant amounts appeared first on the 22nd day. Kidney activity showed an optimum at pH 7.0, similar to liver. Separation of isozymes of phenylalamine hydroxylase from rat kidney will permit us to determine if the properties of the kidney enzyme are similar to those of the liver enzyme.